Macroemulsions for use in antifreezes are well-known in the art. For example, ethylene glycol antifreeze compositions containing mineral oil plus an emulsifier or containing a barium sulfonate oil are well-known and were used commercially in automobiles up until the mid-1960's. The oil in these antifreezes provided excellent corrosion inhibition for the antifreezes by coating the metal surfaces of the coolant system with a so-called "magnetic film". However, in concentrate form, these emulsions are generally phase stable only for a very short period of time, e.g., perhaps two days or less, after which time they tend to separate into two phases.
With the advent of antifreeze overflow-tanks used to hold the volume expansion of antifreeze in a hot engine coolant system, these rather unstable macroemulsion antifreezes were replaced with other types of antifreezes, since a macroscopic two-phase antifreeze tends to congregate the less dense oil phase in the overflow tank thereby depriving the remaining parts of the cooling system of this corrosion-inhibiting component. For the above described reasons, the concept of macroscopic two-phase antifreezes is obsolete and current antifreezes are single phase, most notably the silicone silicate stabilized antifreezes disclosed in U.S. Pat. Nos. 3,337,496 and 3,341,469.
In recent years, microemulsions have been developed which are known for their long term storage stability, i.e. their ability to maintain an apparent single phase over an extended period of time. By way of illustration, oil-in-water microemulsions and water in oil microemulsions are extensively employed in the hydraulic fluid field. For example, U.S. Pat. No. 4,337,161 discloses oil-in-water microemulsions whereas U.S. Pat. No. 4,371,447 discloses water-in-oil microemulsions for use in hydraulic fluids, and the like.
Microemulsions, variously referred to as micellar solutions, "soluble oils", swollen micelles, etc., have been extensively discussed in the literature. See, for example, Microemulsions, I. D. Robb, Editor, Plenum Press (1981); and, Microemulsions Theory and Practice, L. M. Prince, Editor, Academic Press (1977). Microemulsions are typically characterized as clear, bright and transparent (and, hence, apparently homogeneous and microdispersed), whereas macroemulsions are visibly turbid and two phase. These characteristics are due to the fact that the particle (i.e. micro droplet) in a microemulsion is typically smaller than the wave-length of visible light. Although clear to the eye, the presence of the microemulsion particles can be detected by instrumental light-scattering techniques.
As used herein, the term "microemulsion" is used in its broadest sense to encompass faintly, turbid or opalescent compositions as well as completely clear ones. These microemulsion compositions are characterized by excellent ambient temperature storage stability and particle sizes of less than about 0.1 micron. The microemulsions are physically stable and do not phase separate upon standing at ambient temperature. In contrast, macroemulsions are turbid in appearance (e.g., milk) and tend to phase separate upon standing at ambient temperature.
Although the microemulsion, and the components thereof, present in oil-in-alcohol-containing antifreezes generally provides useful corrosion-retardation on cooling system metal surfaces, certain microemulsions, and the components thereof, provide less than optimum metal corrosion inhibition. Accordingly, it would be desireable to enhance the corrosion inhibition provided by antifreezes containing oil-in-alcohol microemulsions.
Heretofore, microemulsions in antifreeze, containing oil-soluble corrosion inhibitors, were not known in the art to the best of our knowledge. A possible reason for this absence is the conflicting requirements of such a composition. If the antifreeze ingredients are in stable, microemulsion form, there is a problem as to how the oil-soluble corrosion inhibiting components thereof will be made available to protect the metal surfaces of the car's coolant system.